Electricity: conductors and insulators – Conduits – cables or conductors – Preformed panel circuit arrangement
Reexamination Certificate
1998-01-30
2003-03-11
Paladini, Albert W. (Department: 2827)
Electricity: conductors and insulators
Conduits, cables or conductors
Preformed panel circuit arrangement
C174S258000
Reexamination Certificate
active
06531663
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to electrical connections for surface-mount circuit components of hybrid circuits. More particularly, this invention relates to a thick-film solder stop for a solder joint of a surface-mount component, in which the solder stop has a composition that promotes the thermal cycle fatigue resistance of the electrical connection.
BACKGROUND OF THE INVENTION
Flip chips, ball grid arrays (BGAs), wire bond pads, chip resistors and chip capacitors are examples of surface-mount devices, i.e., discrete circuit devices mounted to the surface of a circuit board, such as a printed circuit board (PCB), ceramic substrate, printed wiring board (PWB), flexible circuit, or a silicon substrate. These devices rely on solder joints to both secure the chip to a circuit board and electrically interconnect the device to conductors formed on the circuit board. The size of a flip chip is generally on the order of a few millimeters per side, while bond pads, chip capacitors and resistors are typically smaller. As a result, the conductors required for surface-mount devices are narrow, e.g., line widths of about 0.5 millimeter or less, and typically spaced apart about 0.5 millimeter or less.
Because of the small size of the solder joints, soldering a surface-mount device to its conductor pattern requires a significant degree of precision. Reflow solder techniques are widely employed for this purpose, and typically entail precisely depositing a controlled quantity of solder using methods such as printing and electrodeposition. For smaller surface-mount devices, such as chip resistors and capacitors, the chip is soldered to its conductors by registering terminals formed on the chip with solder deposited on the conductors, and then reheating, or reflowing, the solder so as to form a “solder column” that metallurgically adheres and electrically interconnects the chip to the conductors, yielding a solder joint. Mounting of flip chips and BGAs differ in that the solder is typically deposited on bond pads on the chip. Thereafter, the chip is heated above the liquidus temperature of the solder to yield “solder bumps.” After cooling to solidify the solder bumps, the chip is soldered to the conductor pattern by registering the solder bumps with their respective conductors and then reflowing the solder, again forming solder joints.
Placement of the chip and reflow of the solder must be precisely controlled not only to coincide with the spacing of the terminals and the size of the conductors, but also to control the orientation of smaller surface-mount devices and the height of flip chip solder joints after soldering. As is well known in the art, smaller chips are prone to twisting and tilting during reflow as a result of the device floating on the surface of the molten solder, while controlling the height of flip chip solder joints after reflow is often necessary to prevent the surface tension of the molten solder bumps from drawing the flip chip excessively close to the substrate during the reflow operation. Sufficient spacing between a flip chip and its substrate, which may be termed the “stand-off height,” is desirable for enabling stress relief during thermal cycles, allowing penetration of cleaning solutions for removing undesirable processing residues, and enabling the penetration of mechanical bonding and encapsulation materials between the chip and its substrate.
The position and height of a solder column of a discrete component are generally controlled by limiting the surface area over which the printed solder is allowed to reflow. As illustrated in
FIG. 1
, which shows a conductor
12
in longitudinal cross-section, the latter approach typically involves the use of a solder stop
14
, which is typically formed by a solder mask or printed dielectric. The solder stop
14
extends widthwise across the surface
18
of the conductor
12
, which is printed or otherwise formed on a dielectric substrate
10
, such as alumina. A solder joint
16
is shown as joining a surface-mount (SM) component
20
to the surface
18
of the conductor
12
, as would be the case after solder has been printed and reflowed on the conductor
12
, and the component
20
then registered and reflow soldered to the conductor
12
. As is apparent from
FIG. 1
, the solder stop
14
delineates an area on the surface
18
of the conductor
12
over which solder is able to flow during reflow to form the solder joint
16
. By properly locating the solder stop
14
on the conductor
12
, the degree to which the molten solder can spread during reflow is controlled, which in turn determines the height of the solder joint
16
and therefore the stand-off height of the component
20
relative to the substrate
10
.
Because solder is registered and soldered directly to the conductor
12
, the conductor
12
must be formed of a solderable material, which as used herein means that a tin, lead or indium-based alloy is able to adhere to the conductor
12
through the formation of a metallurgical bond. In contrast, the solder stop
14
is intentionally formed of a nonsolderable material, meaning that solder will not adhere to the material for failure to form a metallurgical bond. Upon reflow, the reflow area defined by the solder stop
14
on the conductor
12
causes the solder joint
16
to have a columnar shape between the component
20
and the conductor
12
.
Though widely used in the art, trends in the industry have complicated the ability for solder stops to yield solder joints that exhibit adequate reliability. Particularly, the trend is toward the use of low-melting, high-tin (e.g., 60Sn—40Pb) solder that is relatively brittle. Thermal cycle reliability problems can occur when a brittle solder solidifies against a solder stop used to contain the solder during reflow. During thermal cycling, fatigue fractures
22
tend to occur in the conductor
12
at the junction between the solder joint
16
and solder stop
14
, as shown in FIG.
1
. The cause of the fracture
22
is generally the mismatch of the coefficients of thermal expansion (CTE) of the conductor
12
, solder stop
14
and solder. Solder stops are typically a hard thick-film dielectric material having a CTE roughly equal to that of the alumina substrate (about 6.7×10
−6
/° C.), while the CTE of the solder is typically much higher—e.g., about 25×10
−6
/° C. for lead-tin solders. The CTE mismatch is further exasperated by surface-mount components whose CTEs are typically about 4×10
−6
/° C. to about 25×10
−6
/° C. The resulting stresses developed in the joint during thermal cycling due to the large CTE mismatch are thought to be intensified or concentrated at the solder-conductor-solder stop interface, where the solder is inhibited by the hard solder stop material. Eventual stress relief is achieved through the creation of a crack through the underlying conductor, as shown in FIG.
1
. The fracture occurs in the conductor because the conductor is the weakest structure at the solder-conductor-solder stop interface, and is therefore more susceptible to fracture by thermal cycle fatigue than the solder joint and solder stop.
To reduce the occurrence of fatigue fractures in conductors, solder stops formed of nonconductive polymer materials have been used that absorb the stresses created by the highly expanding solder. However, suitable polymers exhibit inferior printing characteristics and require special curing processes that are not conducive to thick-film manufacturing processes. Accordingly, it would be desirable if an improved solder stop were available that was capable of diverting or absorbing thermal stresses generated by the CTE mismatch at the conductor-solder stop interface.
SUMMARY OF THE INVENTION
It is an object of this invention to provide an electrical connection comprising a conductor, a solder joint, and a solder stop that inhibits fracturing of the conductor and solder joint during thermal cycling.
It is another object of this invention that the solder stop has a
Berlin Carl William
Ferraro William Paul
Isenberg John Karl
Myers Jay Robert
Delphi Technologies Inc.
Funke Jimmy L.
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